class Circle:
    def __init__(self,radius):
        # 半径
        self.radius = radius
        # 私有化圆周率属性 
        self.__PI = 3.1415926


    def perimeter(self):
        '''
        求圆的周长
        :return: 圆的周长
        '''
        return 2 * self.__PI * self.radius
    
class Person:
    def __say_hello(self):
        print('hello')